If they made a halo 2 anniversary I would hope for the following:
Remastered campaign with 100% true gameplay, original skulls( but with a menu in the lobby to actually turn them on and off at will), a delta halo archive(terminals) and ability to switch back and forth between new and old graphics.
Unlockable E3 demo mission and maybe another mission that was cut from the game. The demo mission being unlocked by beating the game and the cut mission being unlocked by doing something like finding all the skulls.
A map pack for whatever other halo game is most recent(halo 4,5, or 6)
Halo 2 multiplayer arcade that would be released the same day as halo 2 anniversary in the xbox live arcade. Halo 2 anniversary could come with a code that entitles you to a free download of this.
So those are some of my ideas. What would you guys like to see in a Halo 2 anniversary?